RTAK ii 1075 now?

You are correct SK-5 is very similar to 1080. Carbon (SK-5 0.8-0.9%, 1080 0.75-0.88%) and manganese content (SK-5 0.1-0.5%, 1080 0.6-0.9%) are very similar and SK-5 also has small amounts of nickel, chromium, copper and silicon. The performance of both steels should be pretty much identical but the 1080 with the higher manganese content would have a slightly slower critical cooling rate and therefore react better to heat treatment if the critical cooling rates are marginal. That said I believe the problem with the blade is as you surmised- that it is a heat treating, not a steel, problem.
 
Dan Maragni said:
Let me do a little research and I will get back to you.
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Hi Dan, thanks for your detailed response.

Is the progression of brittleness between 1055 and 1095 linear, or is there a critical point when the properties suddenly become quite different?

Also, what is your opinion on 1055?

I have machetes in 1055 that are quite soft from one manufacturer, and other 1055 machetes from another manufacturer that rival the excellent 1075 blades made by imacasa.
 
Mr. pangaman- Lath martensite forms in steel at 0.6% carbon and below and plate martensite forms in steels having 1.0% carbon and above. In between a mixture of lath and plate forms with more plate and less lath as you approach 1.0% carbon. 0.6% carbon is about all the carbon that will dissolve in the matrix (which is dependent on a variety of factors) and excess carbon will form iron carbides (in a simple carbon steel). These carbides can cause brittleness in steel.

As steel is hardened (transformed to martensite) the martensite will grow and the higher the carbon content the larger the growth factor. In higher carbon steels this will commonly cause micro-cracks as the plates impinge on each other which will also increase brittleness in steel.

1055 will not "fully harden" the matrix and you may find the blades to be slightly malleable but the "softness" problem is probably due to poor heat treatment.

A great reference for anyone with questions about heat treating blades is Dr. Larrin Thomas' "Knife Engineering: Steel, Heat Treating, and Geometry". Dr. Thomas has bridged the gap between "metallurgy" and "blade metallurgy" something I have been trying to do throughout my career. His book on "The Story of Knife Steel: Innovators Behind Modern Damascus and Super Steels" is also very well researched and worth checking out.
 
Dan Maragni said:
Mr. pangaman- Lath martensite forms in steel at 0.6% carbon and below and plate martensite forms in steels having 1.0% carbon and above. In between a mixture of lath and plate forms with more plate and less lath as you approach 1.0% carbon. 0.6% carbon is about all the carbon that will dissolve in the matrix (which is dependent on a variety of factors) and excess carbon will form iron carbides (in a simple carbon steel). These carbides can cause brittleness in steel.

As steel is hardened (transformed to martensite) the martensite will grow and the higher the carbon content the larger the growth factor. In higher carbon steels this will commonly cause micro-cracks as the plates impinge on each other which will also increase brittleness in steel.

1055 will not "fully harden" the matrix and you may find the blades to be slightly malleable but the "softness" problem is probably due to poor heat treatment.

A great reference for anyone with questions about heat treating blades is Dr. Larrin Thomas' "Knife Engineering: Steel, Heat Treating, and Geometry". Dr. Thomas has bridged the gap between "metallurgy" and "blade metallurgy" something I have been trying to do throughout my career. His book on "The Story of Knife Steel: Innovators Behind Modern Damascus and Super Steels" is also very well researched and worth checking out.
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Thanks Dan, will definitely check out Dr Larrin Thomas.
 
pangaman said:
Hi Dan, what are thoughts on SK5 which I believe is roughly equivalent to 1080, and
I therefore imagine be pretty close to 1075 if properly heat treated?

I have a cold steel bushman that lost a finger nail sized chip out of the cutting edge, and was wondering if it was just a random QC issue relating to the heat treat. The steel seemed quite granular around the chip.

Thanks for the informative articles!
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Some of the Cold Steel Bushman were/are made in China. That might explain a poor heat treat. Taiwanese SK-5 is great steel. They learned how to make quality steel from the Japanese. I'm wary of Chinese steel. I'm sure there are some good Chinese steel companies, but there are also a lot of bad ones. I personally own one Cold Steel Chinese manufactured knife, the Large Luzon. It's 8Cr13MoV stainless steel. It's my EDC. I'll say this, it holds an edge extremely well. It's still hair popping sharp after like 4 years of carrying it. I continue shaving hairs with it, and it never gets dull. My genuine Ka-Bar literally got full, merely from shaving hairs! I wouldn't trust it for any type of moderate abuse, but that's not what the Large Luzon is made for. It's a pure self defense knife, and it fills that role better than any other folder I've seen. It's light, fast, and has a long handle for slashing or choking back. I love how on Cold Steel's website, it doesn't mention that it's made in China, yet it says China right on the blade! 😂
 
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